Time of flight (TOF) mass spectrometers are widely used to determine the mass-to-charge ratio (m/z) of ions on the basis of their flight time along a flight path. Ions are emitted from a pulsed ion source in the form of a short ion pulse and are directed along a prescribed flight path through an evacuated space to reach an ion detector. The ion source is arranged so that the ions leave the source with a constant kinetic energy and therefore reach the detector after a time which depends upon their mass, more massive ions being slower. The detector then provides an output to a data acquisition system and a mass spectrum can be constructed. The present invention is applicable to such TOF mass spectrometry amongst other forms of mass spectrometry.
In modern time-of-flight (TOF) mass spectrometry, multiple-reflection TOF (MR TOF) systems employing ion mirrors are known as one of the ways to improve resolving power without increasing greatly the size of an instrument. This is achieved by an increase of the ion path length in such systems. However, the performance of MR TOF instruments is limited mainly by the ion optical properties of the ion mirrors. Thus, it is especially important to develop a robust, reliable and simplified mirror design enabling high resolving power as well as high transmission of ions. In addition, it is important to minimise potential space charge effects which would otherwise limit the dynamic range of the MR TOF instrument.
Many proposals for MR TOF, for example, as described in U.S. Pat. No. 3,226,543, U.S. Pat. No. 6,013,913, U.S. Pat. No. 6,107,625, WO 02/103747, WO 2008/071921, have utilised multiple reflections between two coaxial ion mirrors. However, this geometry severely limits the mass range of the analysis due to overlap of ions of different mass-to-charge ratio after a certain number of reflections.
Multiple-reflection ion mirrors for time-of-flight mass spectrometry without mass range limitation have been described by H. Wollnik in GB 2,080,021. In Wollnik's design, each mirror typically provides one reflection and the mirrors are presumed independent and could have either planar or cylindrical symmetry. This construction requires ion trajectories with a large angle of incidence at the ion mirrors and the whole system is complex.
Another multiple-reflection TOF design has been proposed in SU 1,725,289 by Nazarenko, wherein two opposing elongated planar mirrors allow multiple reflections of ions between them together with displacement along the direction of mirror elongation (“shift direction”, Z). Though such a construction is simple and allows ion focusing in the two directions other than Z, unlimited divergence of the ion beam along Z limits the mirror performance when used with modern ion sources.
The problem of de-focusing in the Z-direction in the Nazarenko geometry has been addressed by A. Verentchikov et al. in WO 2005/001878, wherein a design is described having additional planar lenses periodically positioned in the space between the opposing elongated mirrors so that the ion beam is repetitively focused as it spreads along Z. Such mirrors have also been proposed for use in tandem mass spectrometry (US 2006/0214100 A, US 2007/0029473 A). High resolving power of such mirrors has been demonstrated experimentally. However, the focusing by the lenses remains relatively weak in comparison to focusing in other directions which limits the acceptance of the analyser. Also, the location of lenses in the middle of the mirror assembly complicates the implementation of the design. For example, it restricts the location of any detector(s) in the same plane, which normally coincides with the plane of time-of-flight focusing of the mirrors, and necessitates an additional isochronous ion transfer as shown in US 2006/0214100 A.